Mapping of the spatial distribution of silver nanoparticles in root tissues of Vicia faba by laser-induced breakdown spectroscopy (LIBS)

The manuscript presents a procedure for optimal sample preparation and the mapping of the spatial distribution of metal ions and nanoparticles in plant roots using laser-induced breakdown spectroscopy (LIBS) in a double-pulse configuration (DP LIBS) in orthogonal reheating mode. Two Nd:YAG lasers were used; the first one was an ablation laser (UP-266 MACRO, New Wave, USA) with a wavelength of 266 nm, and the second one (Brilliant, Quantel, France), with a fundamental wavelength of 1064 nm, was used to reheat the microplasma. Seedlings of Vicia faba were cultivated for 7 days in CuSO4 or AgNO3 solutions with a concentration of 10 mu mol 1(-1) or in a solution of silver nanoparticles (AgNPs) with a concentration of 10 mu mol 1(-1) of total Ag, and in distilled water as a control. The total contents of the examined metals in the roots after sample mineralization as well as changes.in the concentrations of the metals in the cultivation solutions were monitored by ICP-OES. Root samples embedded in the TissueTek medium and cut into 40 gm thick cross sections using the Cryo-Cut Microtome proved to be best suited for an accurate LIBS analysis with a 50 gm spatial resolution. 2D raster maps of elemental distribution were created for the emission lines of Cu(I) at 324.754 nm and Ag(I) at 328.068 nm. The limits of detection of DP LIES for the root cross sections were estimated to be 4 pg for Cu, 18 pg for Ag, and 3 pg for AgNPs. The results of Ag spatial distribution mapping indicated that unlike Ag+ ions, AgNPs do not penetrate into the inner tissues of Vicia faba roots but stay in their outermost layers. The content of Ag in roots cultivated in the AgNP solution was one order of magnitude lower compared to roots cultivated in the metal ion solutions. The significantly smaller concentration of Ag in root tissues cultivated in the AgNP solution also supports the conclusion that the absorption and uptake of AgNPs by roots of Vicia faba is very slow. LIBS mapping of root sections represents a fast analytical method with sufficient precision and spatial resolution that can provide very important information for researchers, particularly in the fields of plant science and ecotoxicology.